Ranging systems such as the United States' Global Positioning System (GPS) and the Russian Global Navigation System (GLONASS) allow a user to precisely determine his/her latitude, longitude and elevation. Receivers accomplish ranging by decoding precisely-timed ranging signals transmitted by a group of earth-orbiting satellites. These signals are modulated with a unique, high frequency pseudorandom noise (PRN) code, and the receiver uses the timing of these codes to determine the transmission time, and thus the distance, from each satellite.
In the ideal communications channel only the direct, or line-of-sight, signal is present at the receiver. The receiver correlates the line-of-sight signal with a locally generated PRN code and uses the correlation function peak to precisely determine the transmission time. In general, reflections of the transmitted signals may also be received. Specifically, the signals may also arrive at the receiver antenna after reflection from the earth's surface and from various man-made structures. These signals, known collectively as multipath, are combined with the line-of-sight signal at the receiver.
The time delay of multipath signals relative to the line-of-sight signal can result in a degradation of ranging accuracy. Specifically, it may distort the correlation function, thereby shifting the apparent peak of the function. The severity of the resulting inaccuracy is a function of the satellite and receiving antenna positions relative to the various reflecting objects. Methods have been developed to counter the effect of multipath on ranging system performance. For example, U.S. Pat. No. 5,495,499 describes a narrow correlator that reduces the effect of multipath on the range estimate, and U.S. Pat. No. 5,615,232 discloses an iterative curve-matching mechanism to largely eliminate the multipath contribution to the correlation function.
These methods have various limitations and restrictions in resolving the effect of multipath, however, and the user of a GPS positioning system may not be aware of the extent to which multipath interference is degrading the accuracy of the range estimates. For example, a difference in the timing and carrier phases of the line-of-sight signal and the multipath signals may exist, even when distortion of the shape of the correlation function is minimal. The difference results in a shift of the correlation peak, and an associated degradation of the ranging accuracy. This condition may occur when the multipath is produced by "near" reflectors, i.e. when the differences in the relative path lengths of the line-of-sight and multipath signals are less than a significant fraction of a PRN chip.
What is needed is a simple and low-cost system for characterizing errors in a received signal due to multipath interference, in particular, when the interference is produced by "near" reflectors. The results can be used to provide the user with an indication of received signal quality and the presence of multipath. The system should not unduly increase the cost or size of currently available receiving units.